The present invention relates in general to RF communication systems, and is particularly directed to a composite antenna reflector architecture containing an interior solid reflector region, which is adjacent at its perimeter to a generally ring-shaped or annular reflector. The interior solid region is effectively totally reflective to incident RF energy, while the annular reflector is formed of a plurality of regions containing frequency selective surfaces having respectively different partial reflectivities, that are effective to reduce selected sidelobe energy in the overall radiation profile of the antenna. A frequency selective surface is configured of a laminate of layers containing different elements that resonate at different frequencies, spectrally spaced so as to provide at least one composite frequency response characteristic.
As described in the above-referenced ""978 patent, in order to provide simultaneous RF illumination coverage of multiple adjacent terrestrial regions or xe2x80x98spotsxe2x80x99, such as the (forty-four) oval regions of the beam pattern coverage map of the United States of FIG. 1, a geostationary satellite based antenna system typically contains a limited number of xe2x80x98re-usedxe2x80x99 antenna subsystems, operating at different sub-band spectral segments of an available RF bandwidth, and illuminating multiple spatially separated terrestrial regions.
Namely, illumination of all of the spots of the overall terrestrial coverage area is achieved by having the radiation pattern of each antenna subsystem individually pointable to multiple ones of a prescribed subset of spaced apart (not immediately adjacent) terrestrial regions. Thus, for example, illuminating the forty-four spot terrestrial coverage area of FIG. 1 with a four antenna-subsystemxe2x80x94such as that shown in FIG. 2 as being comprised of four antenna transmit-receive pairs A, B, C, D, or eight individual antenna reflectors (and attendant feed horn subsystems)xe2x80x94results in an antenna re-use factor of eleven.
Although the re-use factor may be decreased by reducing the number of (and thereby increasing the area of each of the) illuminated regions, doing so undesirably entails the deployment of larger and increased complexity hardware and/or an increased power specification. Moreover, even though such a multi-antenna system achieves a first level of (spatial) beam-to-beam isolation by illuminating immediately adjacent spots at mutually different sub-bands, there still remains the problem of sidelobe energy spillover of beams illuminating spatially separated regions having the same sub-band. While sidelobe performance and thus associated beam-to-beam isolation may be somewhat improved by tailoring (tapering) the feed horn pattern illuminating the reflector, physical and cost constraints placed on the feed horn structure at relatively high frequencies (e.g., Ku-band and above) effectively limit this approach.
In accordance with the present invention, the above-described beam-to-beam isolation problem is successfully addressed by a composite antenna reflector architecture that is configured to reduce one or more selected sidelobe portions of the overall radiation profile of the antenna spilling over into other regions illuminated regions in the same band. For this purpose, the antenna reflector of the invention contains a generally circular or polygonal, interior solid parabolic or alternately shaped reflector sector or region, that is adjacent at its perimeter to a generally ring-shaped or annular reflector sector. The interior solid region is effectively totally reflective to incident RF energy, while the annular reflector is formed of a plurality of rings having respectively different partial reflectivities, that alter the illumination taper and thereby reduce selected sidelobe energy, and minimizing degradation in coverage performance and gain slope in the overall radiation profile of the antenna.
The reflectivity profile across the rings may be varied in a number of alternative ways to produce a desired varying reflectivity profile. For example, the reflection coefficient of a respective ring may be fixed across the radius or width of the ring, or the reflection coefficient may be varied between a value of 1 (totally reflective) and 0 (totally transmissive) as a function of the radius from the center of the antenna reflector. Also, the values of the respective reflection coefficients for respective rings decrease in the radial direction outwardly from the central dish, so as to realize a tapered reflection coefficient profile across the composite reflector.
The manner in which a tapered reflectivity may be imparted to a respective ring may be achieved in a number of ways. In a first implementation, the resistivity of each ring may be varied, such as by coating the rings with respective films of differing resistivities. However, changing resistivity to vary a respective ring""s reflection coefficient is less than optimum and undesirable from a practical standpoint in a spaceborne environment, due to the thermal issues introduced by the heat absorption properties of a resistive film.
Pursuant to a preferred implementation, rather than vary its resistivity, the composite resonant frequency response characteristic of each annular ring is selectively defined, so as to be different from that of an adjacent ring. This tailoring of the resonant frequency responses may be accomplished by forming a (low loss) frequency selective surface (FSS) type laminate reflector structure, similar to the dichroic laminate structure described and illustrated in the ""978 patent, and containing a pair of overlapping, spatially parallel partially reflective surface layers containing elements such as slotted tripoles, that resonate at respectively different frequencies.
The physical materials employed in and the internal structure of the dual resonant laminate structure may correspond to those employed in the dichroic composite structure containing two frequency selective surfaces described and illustrated in the ""978 Patent. Also, the composite antenna structure of the present invention may be deployed and supported using a backing structure of the type disclosed in the ""978 Patent.
The dual resonant laminate of the present invention differs from the dichroic composite structure of the ""978 patent in that the respectively different resonant frequencies of the two element-containing layers are relatively close together, spectrally. This spectral offset produces a resultant transmission profile that has generally the same (substantially flat) reflectivity or transmission over a prescribed bandwidth. The composite RF transmission profile of this type of low loss resonant laminate can be increased or decreased by changing the resonant frequency elements of one or both of the loaded layers so as to change the their mutual spectral separation, and thereby achieve a prescribed level of RF reflection over a prescribed RF bandwidth from the composite layer structure.
Pursuant to a further embodiment, each of the respective upper and lower layer portions of the laminate structure may contain multiple sets of different slotted elements to provide a plurality of spectrally separated composite response characteristics within the same multiband frequency selective surface (FSS).
In accordance with a dual band embodiment of the invention, the surrounding annular sector may contain two adjacent sets of (two) partially reflective ring-shaped reflectors that surround the central dish and whose respective reflection coefficients have a first set of values for a first operational band, and a second set of values for a second operational band. As in the dual band architecture of the ""978 patent, the inner radial dimension of the exterior annular ring-containing sector is defined so that the effective aperture or beamwidth of the antenna reflector is the same for each of the two spaced apart (xe2x80x98highxe2x80x99 and xe2x80x98lowxe2x80x99) bands at which the antenna is intended to operate. This again allows the composite reflector to be coupled with dual-band feeds capable of operating at both spaced apart frequency bands, and produce the same spot beam pattern for both frequency bands.
For low band operation, the two interiormost rings are totally reflective, so as to effectively increase the diameter of the totally reflective central dish portion of the antenna, while the outer two rings have respectively different fixed or constant reflection coefficients that reduce the sidelobes of the antenna. For high band operation, the two outermost rings are transmissive, so as to effectively decrease the effective diameter of the antenna, while the two interiormost rings have respectively different fixed or constant reflection coefficients for selective sidelobe reduction.